The present invention concerns improvements in the manufacture of anodes for capacitors and specially for solid-electrolyte capacitors of the type described in U.S. Pat. No. 3,093,883 filed on Apr. 2, 1953, by Haring and Taylor and assigned to Bell Laboratories. The commercial success of capacitors of this type is the best proof of the advantages which they can bring to users. The requirements of the users have increased as technological advances were made by the manufacturers of capacitors. The present invention has essentially for its object to provide improvements with the object of bettering the secondary characteristics of capacitors of the above type and more particularly to reduce derating (i.e. variation of the capacitance as a function of temperature) specially in the case of sub. zero temperatures, and to reduce the dissipation factor. By way of example, medium-voltage (40-50 V) type capacitors, as sold by the Assignee under type number PA 85, show a relative variation of the capacitance value between 25.degree. C and - 55.degree. C of only about 30 % of the typical value for identical capacitors which do not incorporate the improvements of the invention. With the same series of capacitors, a reduction of the dissipation factor of up to 40 % of the typical value is provided by means of the invention.
In accordance with the essential feature of the invention, the sintered anode consists of a film forming containing between 100 and 20,000 ppm. of at least one of the following elements: nitrogen, tungsten, molybdenum, vanadium and hafnium. In accordance with a preferred variant of the invention, the anode is manufactured from a mixture of powders, the first of which is pure, film forming metal, while at least one of the others comprises at least one of the elements nitrogen, tantalum, niobium, tungsten, molybdenum, vanadium and hafnium. Preferably, the grain sizes of the powders consist essentially of the same metal as the first powder and at least one of the above-mentioned elements. In accordance with the characteristics of the capacitor to be obtained and the available powders, the relative proportions of the powders in the mixture may cover a very wide range. The mixture may be employed either directly or after a pre-granulation stage as described in U.S. Pat. No. 3,453,105 filed on Oct. 9, 1967, by one of the inventors and S. Flaks and assigned to the same assignee.